The invention relates to a method and apparatus for the inductive hardening of profiled camshafts by means of a magnetic coupling of energy.
Internal-combustion engines are known which permit a cylinder to be cut-off by a charge cycle valve. Such engines have a camshaft or cut-off camshaft which has a narrow zero cam next to each valve cam, at a close axial distance. The zero cam is constructed as a flange which projects radially from the camshaft, and a cut-off rocker lever moves on its exterior surface or running surface, concentric to the axis of rotation of the camshaft. For this purpose, the running surface must be hardened and ground precisely.
Published documents disclosing cut-off camshafts with so-called concentric zero cams with a diameter that corresponds approximately to the base circle of the cams, or which are larger than the cam tip, are not known to the applicant. However, German Patent Documents DE 44 04 683 C1 or DE 42 13 147 A1 illustrate two differently shaped cams for alternative operation of the respective assigned valve. In particular, the former document is similar to a cut-off camshaft, because its coupling mechanism for the two rocking levers is analogous. In the case of the cut-off camshaft, one of the two cams (the so-called zero cam or cut-off cam) has a circular shape and is arranged concentric to the axis of rotation; it is decisive in the cutoff condition of the cylinder. The cut-off or zero cam simply holds the rocking lever in a substantially lifted-off position relative to the operating cam, so that the rocking lever no longer contacts the operating cam at any circumferential point, and the operating cam can move without contact under the pertaining rocking lever. The associated valve remains closed, and a charge cycle does not take place in the corresponding cylinder.
Known valve cams of cut-off camshafts having one base circle and one cam disk tip respectively are provided for operating the inlet and outlet valves of cylinders of an internal-combustion engine. Each valve is operated by a switch rocking lever which contacts the running surface of the valve cams. The projecting tip of the cam disk, which functions as an eccentric, actuates the switch rocking levers (and thus the valves of the rotating camshaft) during operation of the internal-combustion engine.
A cut-off of cylinders is performed by the cut-off rocking levers which are guided on the running surf aces of the zero cams. During the cut-off of cylinders, the cut-off rocking lever of a valve is connected with the corresponding switch rocking lever. As is known, the switch rocking lever will then no longer be actuated by the valve cam, so that the corresponding valve (and therefore the cylinder) is cut off.
In the case of axially close spacing with very narrow zero cams, which cannot be ground by means of grinding wheels in the plunge-cut grinding process, the diameter of the zero cams is sufficiently large that the zero cam projects slightly over the largest elevation of the valve actuating cam at the cam tip, and can be ground over the valve cams.
In the case of axial spacing which is not as close, it is known to design the zero cams wider, and to grind them in the plunge-cut grinding process. In this case, the diameters of the zero cams are designed to be as small as the base circle of the valve actuating cams, and the valve actuating cams project radially over the zero cams in the circumferential area of the cam elevation.
In both cases, a severe fracturing of the camshaft to be hardened occurs at least on a partial circumference of the camshaft--viewed in the axial direction. Such fracturing presents problems during induction hardening, since the magnetic field emitted by the inductor is coupled unevenly to the camshaft in the fractured area, unevenly heating the camshaft, and causing a non-uniform and unusable hardening.
Moreover, non-uniform heating causes local overheating, so that tension gradients occur in the camshaft during the chilling. These may result in tension cracks on the surface, and failure of the camshaft in the operation.
Furthermore, an undesirable hardening will occur in partial areas of the shaft core, which must have sufficiently high ductility for perfect functioning of the camshaft.
It is therefore an object of the invention to provide an arrangement which achieves a uniform hardness penetration of a camshaft with a profile surface, avoiding excessive tension gradients in the camshaft. Furthermore, it is an object of the invention to provide a process for the inductive hardening of a profiled camshaft which ensures a homogenous heating of the camshaft and thus the avoiding of local overheating.
This object is achieved according to the invention, by an arrangement of dynamo sheets on the axial members of the inductor. A magnetic field emanating from the inductor in the area of the dynamo sheets is focused in the radial direction. As a result, areas of the camshaft which have a smaller diameter can be acted upon by means of a higher magnetic field intensity. Since, by means of this loop shaping of the inductor, the magnetic field lines are situated in a plane which is perpendicular to the camshaft axis, there will be a lower heating of adjacent areas of the camshaft with a larger diameter so that overheating is avoided there.
Advantageously, camshafts with a profiled surface can therefore be acted upon in a targeted manner during the inductive hardening, by a magnetic field which is adapted to the profiled surface, without local overheating in larger diameter areas or of heating which is too weak in areas of smaller diameters.
By virtue of multiple heating, the process according to the invention makes it possible to divert the leading heating of the zero cams into the core of the camshaft. Thus, the valve cams are sufficiently heated without overheating of the zero cams.
Furthermore, as a result of the stepped heating, the deeper areas of the camshaft are also heated, so that tension characteristics in the camshaft which lead to the formation of cracks, can be avoided or reduced.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.